Rearmounted forward-looking radio frequency antenna for projectiles

ABSTRACT

A new radio frequency antenna which is mounted on the rear of the nose cone of a projectile which generates a forward-looking radiation pattern is provided. The antenna comprises the hollow metal nose cone of the projectile with a flat metal base attached. This is electromagnetically excited by an RF source connected to a hollow metal L-shaped cylindrical tube having a wire coaxially extended therethrough. One end of the wire is connected to the radio frequency source along with the hollow tube. The other end of the wire is connected to another similarly shaped solid metal L-shaped cylindrical rod wherein both said Lshaped rod and said L-shaped tube form a U-shaped structure wherein the end faces of said structure are attached to the nose cone. The wire extending through said hollow tube is surrounded by dielectric material and is isolated from said tube throughout. The wire extending through the tube extends beyond the tube and attaches to the rod in such a fashion as to form a gap between the end face of said tube and the end face of said rod. The radio frequency source is attached to the coaxial wire-tube assembly by means of a standard RF connector.

United States Patent [191 Jones, Jr. et al.

[4 1 Oct. 29, 1974 21' Appl. No.: 381,118

[52] US. Cl. 343/708, 343/792 [51] Int. Cl. H0lq 1/28 [58] Field ofSearch 343/711, 712, 713, 792,

[56] References Cited UNITED STATES PATENTS 4/1960 Kandoian 343/7927/1972 Jones et al. 343/708 Primary ExaminerEli Lieberman Attorney,Agent, or Firm-Nathan Edelberg; Saul Elbaum ABSTRACT A new radiofrequency antenna which is mounted on the rear of the nose cone of aprojectile which generates a forward-looking radiation pattern isprovided. The antenna comprises the hollow metal nose cone of theprojectile with a flat metal base attached. This is electromagnetic-allyexcited by an RF source con nected to a hollow metal L-shapedcylindrical tube having a wire coaxially extended therethrough. One endof the wire is connected to the radio frequency source along with thehollow tube. The other end of the wire is connected to another similarlyshaped solid metal L-shaped cylindrical rod wherein both said L- shapedrod and said L-shaped tube form a U-shaped structure wherein the endfaces of said structure are attached to the nose cone. The wireextending through said hollow tube is surrounded by dielectric materialand is isolated from said tube throughout. The wire extending throughthe tube extends beyond the tube and attaches to the rod in such afashion as to form a gap between the end face of said tube and the endface of said rod. The radio frequency source is attached to the coaxialwire-tube assembly by means of a standard RF connector.

10 Claims, 7 Drawing Figures PMENIEUBBIZQIQM v 7 sum 10? 3 REARMOUNTEDFORWARD-LOOKING RADIO FREQUENCY ANTENNA FOR PROJECTILES RIGHTS OF THEGOVERNMENT The invention described herein may be manufactured, used, andlicensed by or for the United States Government for governmentalpurposes without the payment to the inventor of any royalty thereon.

BACKGROUND OF THE INVENTION Antennas of conventional design for use onprojectile nose cones require cutouts and dielectric windows throughwhich the electromagnetic radiation from the antennas mounted inside theprojectile, or at the surface of the projectile, can be transmitted.Until now, this has been the only method by which a forwardlookingradiation pattern could be achieved. For a lightweight, high velocityprojectile the required cutouts for conventional designed antennas forprojectile nose cones are undesirable. These cutouts on the surface ofsaid nose cones structurally weaken the rigidity of the cone. Inaddition, it is well known that dielectric material mounted onprojectile nose cones or forming a part thereof ablates. The ablationrate of the material forming the body of the projectile and the materialforming the dielectric antenna windows in most cases are different.These non-uniform ablation rates are known to cause aerodynamicinstabilities during high speed endoatmospheric flight.

Until now, the idea of locating and mounting a radio frequency antennaon the base of a projectile nose cone and obtaining an adequateforward-looking radiation pattern has been totally rejected. Heretofore,this type of radiation pattern from a base mounted antenna has beenconsidered difficult, if not impossible, to achieve. Since highfrequency electromagnetic radiation tends to propagate along a line ofsight path and since an antenna mounted on the base of a cone cannot beseen when viewing a projectile nose cone head on, little radiation inthe forward direction is expected. In fact, the opposite is normallyexpected; most of the electromagnetic radiation should be directed tothe rear.

A typical approach to obtaining a forward-looking radiation pattern witha rear mounted antenna is to attach a center-fed half wavelength dipoleto the rear of the nose cone. One problem with the center-fed dipole isthat it is inherently unbalanced and requires a balun for properoperation and coupling to the RF source.

Other conventional antennas mounted on the rear of a nose cone typicallygive a front to back power ratio of to l0 dB, that it, a rearwardlooking antenna.

It is therefore an object of this invention to provide a new antennawhich although mounted on the base of a projectile nose cone generatesan excellent forwardlooking radiation pattern.

It is another object of this invention to provide a radio frequencyantenna for projectiles which eliminates the need for cutouts anddielectric windows on the projectile nose cone.

It is another object of this invention to provide a new and novel rearmounted forward-looking radio frequency antenna for projectiles which isinherently balanced and therefore obviates the need for a balun.

It is yet another addition object of this invention to provide a rearmounted forward-looking radio frequency antenna for projectiles which ismounted on a recessed base of the projectile nose cone and therebyprovides thermal protection for the active elements.

It is still another additional object of this inventionn to provide arear mounted forward-looking radio frequency antenna mounted on arecessed base of the projectile nose cone whereby the radio frequencycurrents which contribute to the forward-looking radiation pattern areconcentrated on the trailing edge region of the cone.

Another additional object of this invention is to provide aforward-looking radio frequency antenna whose front-to-back ratio is notaffected by the shape of the dielectric coating on the forward portionof the projectile nose cone.

These and other objects of the present invention will become more fullyapparent with reference to the following specifications and drawingswhich relate to two preferred embodiments of the present invention.

SUMMARY OF THE INVENTION In accordance with the disclosure hereinpresented this invention provides a new and novel forwardlooking radiofrequency antenna for a projectile wherein the active radio frequencyelement is mounted on the rear of the projectile nose cone. The antennacomprises a U-shaped cylindrical structure having the end faces of itstwo arms attached to the metal base of a hollow metal nose cone. TheU-shaped structure consists of an L-shaped solid metal rod and anL-shaped hollow metal tube. The hollow metal tube has a metal wireextending coaxially therethrough. The end termination of the wirenearest the middle of the U-shaped structure is attached to the solidmetal rod. A radio frequency power source is connected across the otherend termination of the wire and the hollow tube by means of aconventional coaxial connector. The invention further comprises a gapbetween the end face of the arm of said tube and the end face of the armof said rod located on the same axis of symmetry. In this gap the wireis exposed whereas the space inside said hollow tube and around saidwire is filled with a dielectric material.

BRIEF DESCRIPTION OF THE DRAWINGS The specific nature of the inventionas well as other objects, aspects, uses, and advantages thereof willclearly appear from the following description and from the accompanyingdrawings, in which:

FIG. la is a perspective view of the rear mounted forward-looking radiofrequency antenna.

FIG. 1b is a cross-sectional view of the excitation element of the rearmounted forward-looking radio frequency antenna showing the radiofrequency source and means for mounting.

FIG. 2a is a perspective view of the rear mounted forward-looking radiofrequency antenna wherein the active radio frequency excitation elementis mounted on a recessed base of the nose cone.

FIG. 2b is a cross sectional view of the forwardlooking radio frequencyantenna wherein the rear mounted radio frequency excitation element isattached to a recessed base on the nose cone.

FIG. 3 is a perspective view of the radio frequency excitation elementof the forward-looking radio frequency antenna showing the detailstructure of the gap connection.

FIG. 4 is a graph of the E and E components of the radiation pattern ofthe antenna.

FIG. 5 is a graph of the E and cross-polarized E component of theantenna radiation pattern.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention disclosed hereinmay be understood with respect to its broad aspects by reference to FIG.la wherein there is shown a perspective view of an embodiment of theantenna. The structure 30 shown in this figure is a projectile nose cone20 comprising a conductive metal and having a conductive metal base 10sealing the rear of said cone 20. In this embodiment the radio frequencyexcitation part of the antenna is bilaterally and axisymmetricallymounted on to the base 10. This radio frequency excitation element is U-shaped and has its arms mounted to the base 10 by means of mountingblocks 11 and 15. The U-shaped excitation member shown generally at 31consists of two L-shaped members 12 and 13. One L-shaped member is asolid metal rod 12 the other L-shaped member is a hollow metal tube 13having a wire 14 extending coaxially therethrough. L-shaped conductivemetal tube 13 is connected to the base 10 by means of connection block15. L-shaped solid rod 12 is connected to base 10 by means of connectionblock 11. The arm of the L- shaped hollow tube 13 not attached to thebase 10 and the arm of the L-shaped solid metal rod 12 are both locatedon the same axis of symmetry and have a gap 32 therebetween. The gap 32exposes a portion of the wire 14.

In FIG. lb is shown a cross section of the antenna displayed generallyat FIG. la. In this figure, the radio frequency source 33 is shownconnected to the hollow metal cylindrical tube 13 by means of standardradio frequency connector 34. The hollow metal cylindrical tube 13 ismounted onto the base 10 by means of mounting block 15 having screws 21attaching the mounting block thereto. The hollow metal cylindrical tube13 is held in place and electrically connected by means of mountingscrew 22 in the side of mounting block 15.

Another mounting block 11 having screws 21 therein is attached to thebase of the nose cone 10. The solid metal cylindrical L-shaped rod 12 iselectrically attached to the base 10 by means of screw 27 located in theside of the mounting block 11. A small hole 43 is at the end face of theunattached arm of the L-shaped solid cylindrical rod 12. This hole isfor receiving the wire 14 and the L-shaped solid rod 12 has a screw 28mounted in the side of said unattached arm for the purpose of securingthe metal wire 14 in said hole. This wire 14 extends coaxially throughthe hollow metal cylindrical L-shaped tube 13 and is connected to radiofrequency source 33 via standard RF connector 34. The wire 14 isisolated from the hollow metal cylindrical L-shaped tube 13 by means ofa plastic dielectric material 23 filling the space inside of said hollowmetal cylindrical L-shaped tube 13 and around metal wire 14.

In FIG. 2a is shown a perspective view of another embodiment of theinvention herein wherein the radio frequency excitation element forexciting the metal nose cone is mounted onto the recessed base of saidcone. The nose cone 40 of this embodiment has arecessed metal base 41.On this base 41 is mounted the U- shaped excitation element comprisingL-shaped members 12 and 13, as previously disclosed, attached by meansof mounting blocks 22 and 27. In FIG. 2b is shown a cross section ofthis specific embodiment of this invention wherein the element forexciting the nose cone is mounted on a recessed base of said nose cone.The only difference between this specific embodiment and that of FIG. 1bis that a new base 41 is recessed within a new nose cone 40. Thedistance of the recession is R; all the various parts and theconstruction for the excitation element are the same. The excitationelement consists of a hollow metal L-shaped cylindrical rod 13 and solidmetal L-shaped cylindrical rod 12, said hollow tube 13 having extendedtherethrough said wire 14. A specific embodiment of this version of theinvention having the dimensions as shown in FIG. 2!; wherein R is thedistance of the recession of the base 41. L is the length of the base ofthe U-shaped excitation member consisting of L-shaped members 13 and 12.X is the dimension of the gap between the L-shaped members 12 and 15. His the length of the arms of the L-shaped members which are attached tothe base 41. D is the diameter of the base of the nose cone 40. For thespecific operating frequency of 261 megaHertz the dimensions for thisspecific recessed version of the invention are as follows:

I) i051) cm x 0.2 cm H l6.() cm R l7.8 cm L 87.0 cm

FIG. 3 is a diagram showing the details of the interconnection betweenthe hollow metal tube 13 and the solid metal tube 12. In this figure thewire 14 is shown mounted within a hole 43 in the end face 44 of thesolid metal cylindrical L-shaped rod member 12 by means of screw 28. Theportion of the wire 14 extending through the hollow metal L-shaped tube13 is surrounded by a dielectric material 23. The dielectric material 23may be any standard dielectric commonly used in the radio frequency artfor coaxial tubing. Typical examples of dielectric material 23 arepolyurethane foam, polyethylene, and Teflon.

FIG. 4 shows both the E 9 and E t plots of the radiation pattern forthis rearmounted forward looking antenna 61. Curve A represents the Eplots in the plane at 0 equal to Curve B represents the E at 0 equal to0. The antenna 61 is oriented along the z-axis and the U-shaped elementis oriented in the x 2 plane. Directive gain for the specific embodimentused for this radiation pattern and having the above specifieddimensions measures 7.2 dB. This is moderately high for an antenna ofsuch simple design as herein presented. The front-to-back ratio of thisversion of the antenna exceeds 35 dB. The L-shaped rod and the L-shapedtube each measure 3.5 mm in diameter. The l millimeter gap between thetwo half sections of the active element comprises an RF voltage gap. Theradio frequency signal is transmitted to this gap through the hollowcylindrical L-shaped tube. This hollow tube and wire assembly with thedielectric material filled therein forms a 50 ohm coaxial transmissionline.

The configuration of the active element of this antenna resembles whatis known in scientific literature as a sleeve dipole. The particulardipole used herein when operated at the second resonance is inherentlybalanced and the current and phase distributions on each half sectionare equal. This inherent balance obviates the need for a balun, which isrequired to balance a center-fed half-wave dipole. Operation of theantenna herein disclosed at the first resonance provides an unwantedinherently high impedence mismatch. Moreover, operation of this antennaat the first resonance tends to provide a backward looking radiationpattern. The U-shaped sleeve type dipole element for this rearmountedconical antenna is therefore most efficiently operated near thefrequency of the second resonance. At this frequency the total length ofthe two half sections, is a full wavelength.

All of the specifications above are for the recessed version of theantenna. The unrecessed version or exposed version of the active elementoperates similarly with only a slight increase in frequency.

The radio frequency currents which contribute to the forward-lookingradiation pattern are mainly concentrated on the trailing edge region ofthe cone. Hence, any unusal shape or distortion, or dielectric coatingon the forward portion of the projectile has little effect on thefront-to-back ratio of the antennas radiation pattern. Only the detailsof the side lobe structure are slightly changed.

This antenna also produces excellent linear polarization. The crosspolarized component is on the average down 27 dB from the level of themain component in the forward direction. In FIG. 5, curve C is a graphof the cross-polarized E field radiation pattern of the antenna hereinpresented. Again, curve B is the E g of the field radiation pattern ofthe antenna.

It is to be understood that the inventor does not desire to be limitedto the exact detail of construction shown and described for obviousmodifications will occur to a person skilled in the art pertaininghereto.

What is claimed is:

l. A forward-looking radio frequency antenna for projectiles comprising:

a. a hollow metal nose cone having a metal base;

b. a solid metal L-shaped cylindrical rod having a first end face and asecond end face, said first end face attached to said base at a firstpoint;

c. a hollow metal L-shaped cylindrical tube having a first open end faceand a second open end face, said first open end face attached to saidbase at a second point;

d. a metal wire attached at one end to said solid rod and extendingthrough said hollow tube;

e. a source of radio frequency power connected across said hollow tubeat said base and the other end of said wire at said base;

f. a dielectric material filling the space inside said hollow tube andaround said wire therein; and

g. the end face of said tube and the end face of said rod being disposedopposite one another across a gap, the arms of said rod and said tubebeing located on the same axis of symmetry.

2. The forward-looking radio frequency antenna of claim 1 wherein saidsecond end face of said rod has a coaxial hole therein for receivingsaid first termination of said wire rod.

3. The forward-looking radio frequency antenna of claim 2 wherein saidfirst point of attachment and said second point of attachment arelocated such that said L-shaped rod and said L-shaped tube arediametrically and bilaterally disposed about the central axis ofsymmetry of said cone.

4. The forward-looking radio frequency antenna of claim 3 wherein saidgap is a radio frequency voltage gap.

5. The forward-looking radio frequency antenna of claim 1 wherein thelength from the first end face of the L-shaped rod to the first end faceof the L-shaped tube is equal to one wavelength of the radiatedelectromagnetic energy from said antenna.

6. The forward-looking radio frequency antenna of claim 5 furthercomprising annular mounting blocks attached to said base and through oneof which extends said rod and through another of which extends saidtube.

7. The forward-looking radio frequency antenna of claim 6 furthercomprising means for rigidly holding said tube and said rod in saidmounting blocks.

8. The forward-looking radio frequency antenna of claim 7 wherein saidwire extends across said gap.

9. The forward-looking radio frequency antenna of claim 8 wherein thebase of said cone is recessed.

10. The forward-looking radio frequency antenna of claim 9 wherein saidcone comprises a trailing edge which extends beyond said base and issubstantially cylindrical.

1. A forward-looking radio frequency antenna for projectiles comprising:a. a hollow metal nose cone having a metal base; b. a solid metalL-shaped cylindrical rod having a first end face and a second end face,said first end face attached to said base at a first point; c. a hollowmetal L-shaped cylindrical tube having a first open end face and asecond open end face, said first open end face attached to said base ata second point; d. a metal wire attached at one end to said solid rodand extending through said hollow tube; e. a source of radio frequencypower connected across said hollow tube at said base and the other endof said wire at said base; f. a dielectric material filling the spaceinside said hollow tube and around said wire therein; and g. the endface of said tube and the end face of said rod being disposed oppositeone another across a gap, the arms of said rod and said tube beinglocated on the same axis of symmetry.
 2. The forward-looking radiofrequency antenna of claim 1 wherein said second end face of said rodhas a coaxial hole therein for receiving said first termination of saidwire rod.
 3. The forward-looking radio frequency antenna of claim 2wherein said first point of attachment and said second point ofattachment are located such that said L-shaped rod and said L-shapedtube are diametrically and bilaterally disposed about the central axisof symmetry of said cone.
 4. The forward-looking radio frequency antennaof claim 3 wherein said gap is a radio frequency voltage gap.
 5. Theforward-looking radio frequency antenna of claim 1 wherein the lengthfrom the first end face of the L-shaped rod to the first end face of theL-shaped tube is equal to one wavelength of the radiated electromagneticenergy from said antenna.
 6. The forward-looking radio frequency antennaof claim 5 further comprising annular mounting blocks attached to saidbase and through one of which extends said rod and through another ofwhich extends said tube.
 7. The forward-looking radio frequency antennaof claim 6 further comprising means for rigidly holding said tube andsaid rod in said mounting blocks.
 8. The forward-looking radio frequencyantenna of claim 7 wherein said wire extends across said gap.
 9. Theforward-looking radio frequency antenna of claim 8 wherein the base ofsaid cone is recessed.
 10. The forward-looking radio frequency antennaof claim 9 wherein said cone comprises a trailing edge which extendsbeyond said base and is substantially cylindrical.